A network of communication stations can share a communication medium (e.g., wires connecting multiple stations or spectrum for transmitting radio signals among stations) using any of a variety of access techniques. Some access techniques (e.g., carrier sense multiple access (CSMA) techniques) include a contention period in which stations contend for use of the medium for transmitting a signal by sensing when the medium is idle. In CSMA techniques, “collisions” sometimes occur when signals from two or more stations overlap. Some CSMA techniques attempt to detect collisions and abort transmission to reduce the negative impact of collisions (e.g., CSMA/CD techniques). Other CSMA techniques include mechanisms to avoid or reduce the probability of collisions (e.g., CSMA/CA techniques). For example, different transmissions may be assigned one of multiple priorities. Access is granted using a Priority Resolution Period in which stations signal the priority at which they intend to transmit, and only the highest priority transmissions are allowed to continue in the contention process. A random backoff mechanism spreads the time over which stations attempt to transmit, thereby reducing the probability of collision.
Some communications over the shared medium are organized as a stream of data (or a “flow” or a “session”) that is transmitted over a period of time at a given average data rate. Some streams are time-sensitive and call for a certain amount of available bandwidth to maintain a desired Quality of Service (QoS). Admission control protocols are used to manage whether and how bandwidth is allocated to different streams being transmitted over the shared medium. Some admission control protocols are managed by a central station in a network. An application that provides data to be streamed from a station (e.g., audio, video, or other multimedia streams) makes a request for a stream to be admitted, which may include information characterizing the stream such as an average data rate and a QoS requirement. A station can determine whether the requested stream is to be admitted or rejected based on current usage of the communication medium. Once a stream is admitted, the transmitting station may still need to contend for usage of the communication medium using an access technique such as a CSMA technique. In some networks there are regular time periods during which stations may contend for usage of the communication medium, and other time periods that are designated as contention-free periods in which time slots are allocated based on a predetermined schedule.